1. Field of the Invention
The present invention relates to a function circuit for converting an input signal into an output signal by a prescribed function. In particular, the invention relates to a function circuit that is less prone to be affected by temperature.
2. Description of the Related Art
FIG. 5 is a circuit diagram of a conventional function circuit. FIG. 6 shows an input/output characteristic of the circuit of FIG. 5.
The function circuit of FIG. 5 is composed of three resistors R1, R2, and R3, two diodes D1 and D2, and two reference supply voltages V1 and V2. As shown in FIG. 5, the resistor R2, the diode D1, and the reference supply voltage V2 are connected to each other in series and the resistor R3, the diode D2, and the reference supply voltage V1 are also connected to each other in series. The resistor R1 is connected to the resistors R2 and R3. One end of the resistor R1 is an input terminal IN and the other end (connecting point) is an output terminal OUT of the function circuit. The diode D2 is opposite in direction to the diode D1. An input signal Vs is input to the input terminal IN. For example, the reference supply voltage V1 is 2 V and the reference supply voltage V2 is 3 V.
In the input/output characteristic shown in FIG. 6, the horizontal axis represents the input signal Vs that is input to the input terminal IN and the vertical axis represents the output signal Vout at the output terminal OUT of the function circuit. In FIG. 6, each of Vs and Vout is in the range of 0 V to 5 V. As shown in FIG. 6, as the voltage level of the input signal Vs increases gradually, two change points xcex1 and xcex2 where linear lines having different slopes are connected to each other smoothly appear in the vicinity of the voltages 2 V and 3 V (reference supply voltages V1 and V2), respectively. A generally S-shaped curve can be formed that is bent at the change points xcex1 and xcex2 that are in the vicinity of 2 V and 3 V.
The output signal Vout shown in FIG. 5 can be given by the following formulae, where Vd is the forward voltage of the diodes D1 and D2:
When Vsxe2x89xa7V1+Vd (in the vicinity of the high-temperature-side change point),
Voutxe2x89xa1{R2/(R1+R2)}(Vsxe2x88x92V1xe2x88x92Vd)+V1+Vd.xe2x80x83xe2x80x83(1)
When Vsxe2x89xa6V2xe2x88x92Vd (in the vicinity of the low-temperature-side change point),
Voutxe2x89xa1{R1/(R1+R3)}(V2xe2x88x92Vdxe2x88x92Vs)+Vsxe2x80x83xe2x80x83(2)
When V1 less than Vs less than V2,
Voutxe2x89xa1Vsxe2x80x83xe2x80x83(3)
because the output resistance of the function circuit is rendered in a high-impedance state.
FIG. 7 is a circuit diagram of another conventional function circuit. FIG. 8 shows an input/output characteristic of the function circuit of FIG. 7.
The function circuit of FIG. 7 is mainly composed of a first circuit including an npn transistor Q1 and a pnp transistor Q2 and a second circuit including a pnp transistor Q3 and an npn transistor Q4. In the first circuit, the base terminal of the transistor Q1 and the emitter terminal of the transistor Q2 are connected to each other. In the second circuit, the base terminal of the transistor Q3 and the emitter terminal of the transistor Q4 are connected to each other. The emitter terminal of the transistor Q1 and the emitter terminal of the transistor Q3 are connected to each other via resistors R2 and R3 that have the same resistance (R2=R3). One end of a resistor R1 is connected to the connecting point P1 of the resistors R2 and R3. The other end of the resistor R1 serves as an input terminal IN to which an input signal Vs is input. A reference supply voltage V1 (2 V) is applied to the base terminal of the transistor Q2, and a reference supply voltage V2 (3 V) is applied to the base terminal of the transistor Q4. The connecting point P1 also serves as an output terminal OUT.
In the second function circuit of FIG. 7, the potential of the emitter terminal of the transistor Q2, that is, the base potential of the transistor Q1, is set higher than the reference supply voltage V1 (2 V) that is applied to the base terminal of the transistor Q2 by the base-emitter voltage Vbe of the transistor Q2. The potential of the emitter terminal of the transistor Q1 is set lower than the emitter potential of the transistor Q2 by the base-emitter voltage Vbe of the transistor Q1. Therefore, the base-emitter voltage Vbe of the transistor Q2 and the base-emitter voltage Vbe of the transistor Q1 are in a relationship that they cancel out each other. The potential of the base terminal of the transistor Q2 and the potential of the emitter terminal of the transistor Q1 are set identical. As a result, as shown in FIG. 8, the function circuit of FIG. 7 has an input/output characteristic having a curve that is centered at 2.5 V (Vcc/2) and is bent in the vicinity of the reference voltage V1 (change point xcex1) and the reference voltage V2 (change point xcex2).
The output signal Vout is given by the following formulae:
When Vsxe2x89xa7V2,
Voutxe2x89xa1{R1/(R1+R3)}(V2xe2x88x92Vs)+Vsxe2x80x83xe2x80x83(4)
When Vsxe2x89xa6V1,
Voutxe2x89xa1{R2/(R1+R2)}(Vsxe2x88x92V1)+V1xe2x80x83xe2x80x83(5)
When V1 less than Vs less than V2,
Voutxe2x89xa1Vsxe2x80x83xe2x80x83(6)
because both of the transistors Q1 and Q3 are rendered off, that is, they are in a high-impedance state.
However, the function circuit of FIG. 5 uses the diodes D1 and D2. In general, diodes have a characteristic that the forward voltage Vd tends to vary with temperature. As seen from Formulae (1) and (2), the formula representing the output signal Vout includes the forward voltage Vd. Therefore, errors indicated by hatching in FIG. 6 occur in the ranges of Vsxe2x89xa7V1+Vd and Vsxe2x89xa6V2xe2x88x92Vd because the diode forward voltage Vd varies being affected by a temperature variation.
Further, since the voltages of the change points are shifted from the respective reference voltages V1 and V2 by the diode forward voltage Vd, designing should take the forward voltage Vd into consideration and hence is complicated.
On the other hand, in the other function circuit of FIG. 7, in general, since a base current Ib2 flowing through the transistor Q2 and a base current Ib1 flowing through the transistor Q1 are different from each other in magnitude, the base-emitter voltage Vbe2 of the transistor Q2 and the base-emitter voltage Vbe1 of the transistor Q1 may be different from each other in magnitude; a relationship Vbe1xe2x88x92Vbe2=0 does not necessarily hold. That is, the two base-emitter voltages Vbe may not cancel out each other sufficiently. As a result, as hatched in FIG. 8, influences of variations in the transistor base-emitter voltages Vbe due to a temperature variation tend to arise in the ranges of Vsxe2x89xa6V1 and Vsxe2x89xa7V2 though in a lower degree than in the function circuit of FIG. 5.
The present invention has been made to solve the above problems, and an object of the invention is therefore to provide a function circuit that is less prone to be affected by temperature.
The invention provides a function circuit for converting an input signal by a prescribed function, comprising a first transistor; a second transistor; voltage dividing means connected to the first transistor, for dividing the input signal with a prescribed division ratio; a reference voltage source for applying a prescribed reference voltage to a base terminal of the second transistor; and a current mirror circuit that is connected to the first transistor and the second transistor so that the same constant current flows between a collector terminal and an emitter terminal of the first transistor and between those of the second transistor.
For example, a first function circuit is such that the first transistor is a pnp transistor and the second transistor is an npn transistor.
A second function circuit is such that the first transistor is an npn transistor and the second transistor is a pnp transistor.
A function circuit may be formed by using at least one pair of the first function circuit and the second function circuit, at least one first function circuit, or at least one second function circuit.
According to the invention, the use of the current mirror circuit makes it possible to allow the same base current to flow through the paired npn transistor and pnp transistor. Therefore, their base-emitter voltages Vbe can be made identical and can cancel out each other sufficiently even with a temperature variation. As a result, the function circuit is not affected by temperature.